This invention relates to a novel method for eliminating ethanol in an exhaust gas by use of a microorganism. More specifically, the invention relates to a method for eliminating ethanol, comprising passing an exhaust gas, which contains ethanol, etc., through a filter bed holding an ethanol-utilizing microorganism, to bring the ethanol, etc. into contact with the microorganism, thereby decomposing the ethanol, etc. so that the ethanol, etc. may be directly eliminated from the exhaust gas. The invention also relates to an apparatus used in performing this method.
Ethanol is one of the volatile organic compounds which contributes to the destruction of the ozone layer, although not to such a high degree as methane. Thus, it is necessary to suppress the emission of ethanol-containing exhaust gases formed during the manufacturing process for fermentation products, for example, at bakery, soy sauce plants, alcohol production plants, and breweries or distilleries; or those generated, for example, at laundry factories using ethanol as a solvent. This is a crucial task for the protection of the earth""s environment.
Exhaust gas containing ethanol have often been eliminated by methods, such as flame incineration using a boiler or the like, catalytic combustion, adsorption to activated carbon or resin, and dissolution into water by mans of a scrubber or by cooling condensation. Treating methods other than burning have required that ethanol be recovered, and then further subjected to combustion or microbial treatment as a post-recovery step.
These methods, however, have been seriously problematical in terms of strict requirements for the site of construction of facilities generating ethanol-containing exhaust gas; restrictions on options for eliminating methods, due to limited use of utilities (electricity, water, drainage); huge initial investments, and high running costs; huge equipment and land allotments for installation. Furthermore, none of these methods have fulfilled all of the requirements regarding treatment ability, economy, safety, as well as ease of maintenance and management.
Methods for eliminating or decomposing substances by use of microorganisms have been developed for sewage disposal, waste water treatment, environmental repair, e.g., through decomposition of effluent oils into seawater, removal of environmental foul odors, and so on. A bioscrubber, which circulates activated sludge water with a scrubber, is known as a device for directly decomposing and removing ethanol in an exhaust gas by use of a microorganism. However, this bioscrubber device treats ethanol, dissolved in activated sludge water, by means of activated sludge in a huge aeration tank mounted in succession to the scrubber. Introduction of this device has required huge equipment and a significant investment.
Another device for eliminating exhaust by use of a microorganism is a biofilter. The biofilter is a device in which an exhaust gas to be treated is passed through a microorganism immobilized in a filter material, so that a target substance is treated by the action of the microorganism in the filter bed. The advantage of this device is that it is more compact and lower in running costs than ordinary activated sludge treatment. However, the existing biofilter has been effective only when the concentration of the target substance to be treated, which is contained in the exhaust gas, is as low as, say, several hundred ppm or less. Such a biofilter with a limited power has been commercially unfeasible.
The method for elimination or decomposition using a microorganism has posed problems, such that it cannot be performed continuously for a long term, it lacks a stable treating ability, and the apparatus involved has complicated maintenance requirements.
Some reports have been issued on the existing techniques for the biofilter that decomposes or eliminates ethanol contained in an exhaust gas. The main information from the reports is offered below.
In the present specification, the ethanol load capacity refers to the amount (weight) of ethanol introduced into the biofilter device per unit time and per unit volume of the filter bed. The ethanol removal efficiency is the rate (percentage) of {[the amount of ethanol (absolute amount) introduced into the biofilter device] minus [the amount of ethanol (absolute amount) in the exhaust gas after discharge from the biofilter device]} to [the amount of ethanol (absolute amount) introduced into the biofilter device].
Hodge et al. reported a test conducted on a small scale, i.e., a laboratory scale (D. S. Hodge and J. S. Devinny, Environmental Progress, Vol. 13, 167-173 (1994)). A cylindrical column with a volume of 4 liters (7.6 cm in diameter by 90 cm in length) was charged with activated carbon as a filter material, and soil from a petroleum refinery land firm was used as a source of a microorganism. When a gas was passed at an ethanol load capacity of 71 to 245 g/m3/hr, the ethanol removal efficiency was 72 to 89%. However, the test results were not confirmed for a long-term continuous operation.
Kiared et al. also reported a test conducted on a small scale, i.e., a laboratory scale (K. Kiared, L. Bibeau, R. Brzeinski, Environmental Progress, Vol. 15, 148-152 (1996)). A cylindrical column with a volume of 21 liters (15 cm in diameter by 120 cm in length) was charged with peat as a filter material, and a mixture of Bacillus was used as a source of a microorganism. When a gas was passed at an ethanol load capacity of 135 g/m3/hr, the ethanol removal efficiency was 88%. However, these results were obtained in a continuous operation performed for up to 30 days.
As large scale, i.e., plant scale, experiments, pilot experiments for elimination of exhaust gas at a bakery have been reported (G. Leson et al., Proceedings of the 86th Annual Meeting of Air and Waste association, 93-WP-52C. 04, pp. 1-14 (1993)). Compost was used as a microorganism source in a reactor with a capacity of 1.42 m3 (1 m in diameter by 1.83 m in height with 2 layers arranged in series). This device was operated, with a gas being passed as a downward flow. At load capacities of 70 g/m3/hr or less, the ethanol removal efficiency was 100%. At load capacities in excess of 100 g/m3/hr, the ethanol removal efficiencies were less than 90%. When the load capacities were 300 g/m3/hr or more, the ethanol removal efficiencies were reported to be 50% or less. These results were also obtained in a continuous operation lasting for a short term of less than 2 months.
Leson et al. similarly reported testing of a full-scale biofilter for elimination of exhaust gas containing ethanol (G. Leson et al., Proceedings of the 88th Annual Meeting of Air and Waste association, 95-WP-9A. 04, pp. 1-11 (1995)). They conducted experiments for treating a foundry exhaust gas, which contained ethanol at a flow rate of 17,000 m3/hr, by a biofilter device with a filter bed volume of 280 m3. In these experiments, a mixture of wood chips, agricultural waste, and manure was used as a filter material.
The treating ability in the initial stage of operation was reported to be the ethanol removal efficiency of 80 to 90% on an average of an ethanol load capacity of 100 to 180 g/m3/hr. No report of long-term operation was made, so that the results of long-term operation, with a constantly high ethanol removal efficiency being maintained stably, are unknown.
Shim et al. reported a test of ethanol decomposition using a spiral type bioreactor having activated sludge immobilized therein (J. S. Shim et al., J. Chem. Tech. Biotechnol., Vol. 64, 49-54(1995)). They reported that 99% ethanol was removed from an exhaust gas containing 7,000 ppm ethanol at a maximum load capacity of 185 g/m3/hr (unit reactor volume). However, they did not carry out a long-term operation test.
Except for the report by Shim et al., all the reports showing the ethanol removal efficiency of 90% or more are concerned with the treatment of an exhaust gas having a low concentration of ethanol as indicated by the ethanol load capacity of 100 to 180 g/m3/hr. Further these reports did not include the results of operation performed continuously for a long term of 3 months or more. That is, there has been no development of a biofilter for decomposition or elimination of ethanol in an exhaust gas, the biofilter that ensures sufficient decomposition or elimination at a high ethanol load capacity of 180 g/m3/hr or more, that permits a long-term continuous operation with this treating ability being maintained, and that can be put to practical use on a plant scale.
Under these circumstances, the inventors of the present invention accomplished a novel method for eliminating ethanol in an exhaust gas by use of a microorganism, as means of decomposing an ethanol-containing exhaust gas formed during the manufacturing process for fermentation products, for example, at bakery, soy sauce plants, alcohol production plants, and breweries or distilleries; or generated, for example, at laundry factories using ethanol as a solvent. The novel method is low in running costs, obviates the need for devices or equipment requiring a heavy initial investment, is more convenient in device maintenance, is easy to handle, can be continuously performed for a long term of more than a half year, is able to decompose ethanol in an exhaust gas having a high concentration of ethanol and a high load capacity, and is high in ethanol removal efficiency, and elimination can be performed at a high speed.
The method of the present invention comprises passing an ethanol-containing exhaust gas through an ethanol-utilizing microorganism held in a filter bed, to bring the ethanol into contact with the microorganism, thereby decomposing the ethanol in the exhaust gas directly by the microorganism. This method permits rapid elimination of ethanol. The present invention also provides a eliminating method which can decompose acetaldehyde, ethyl acetate, and acetic acid, along with ethanol, by the microorganism, even when the exhaust gas contains acetaldehyde, ethyl acetate, and/or acetic acid, in addition to ethanol.